igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 (Mohs scale); Color: Typically black, but can be brown, green, or gold; Luster: Vitreous (glassy); Crystal structure: Amorphous (no crystal structure); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Typically black, but can be brown, green, or gold
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Typically black, but can be brown, green, or gold; Luster: Vitreous (glassy); Crystal structure: Amorphous (no crystal structure); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed by the extremely rapid cooling of silica-rich lava, preventing crystal growth. Usually found in volcanic regions of geologically young ages (Cenozoic) because glass eventually devitrifies or alters.
Uses & applications
Historically used for arrowheads and knives; currently used as a gemstone in jewelry, decorative ornaments, and specialized surgical scalpel blades.
Geological facts
Because it is glass, obsidian breaks with very sharp edges. It is often called 'Nature\'s Glass' and has been traded by humans for thousands of years over long distances.
Field identification & locations
Identify in the field by its very smooth, shiny surfaces and curved 'conchoidal' fractures that look like ripples in glass. Commonly found in Oregon, Mexico, and Iceland.
